Conventional lumbar fusion surgery may result in excessive displacement at adjacent segments other than a surgical site after the operation, resulting in secondary lesions at the adjacent segments during a long-term use and reoperation is often required. In particular, when a problem occurs in a joint between a sacrum (sacral vertebrae) and an ilium (pelvis), a sacroiliac joint syndrome may develop. This syndrome is accompanied by lumbar pain, leading to pain in the thighs and below the hips, and sometimes pain along a sciatic nerve occurs. Further, in a severe case, it has been reported that it becomes difficult to sit.
For the purpose of treating such pains, a sacroiliac (SI) joint fusion implant is used. However, since the sacrum and the ilium are simply fixed using a screw during the operation, loosening occurs at the joint site after a long period of time, and therefore, there is a problem that the reoperation should be performed.
In a method disclosed in Korean Patent Laid-Open Publication No. 10-1999-0077375 developed later, a washer and a bone screw are used by differently forming a diameter of a thread to be transplanted to the segmented bone. However, if rotating the screw until it comes into contact with the washer, the screw may penetrate the bone body located at a region far from the washer of the bone body to be fused, thereby damaging other tissues of a human body. In addition, if tightening the screw only to a position in which the screw does not penetrate the bone body located at the region far from the washer by using an observation equipment, the bone bodies to be fused cannot be sufficiently adhered to each other. Therefore, a length of the screw should be formed within a range so that the bone body to be fused is brought into contact with the washer accurately while fusing the bone bodies to be fused. Accordingly, screws having a required length should be prepared based on differences in the bone structure of each patient, and thus the efficacy of this treatment may be uncertain.